This invention relates to closed-loop control methods and apparatus.
In a closed-loop control system an element is controlled by applying corrective action if a parameter of the element departs from a desired value. Two broad approaches have been used in such systems.
The first uses analogue techniques and adjusts a control input by an amount depending on the magnitude of the error. A disadvantage of such systems is that they tend to be complex, especially if the response of the system is to be tailored, for example by, introducing lag functions or the like to prevent instability.
In the opposite approach the control input has only two values (typically "on" and "off") and control is exercised by switching from one state to the other. Although such systems are relatively simple they tend to be less precise than analogue systems, are prone to overshoot, and are subject to stresses when the control input changes state.